1. Field of the Invention
This invention relates to methods of providing contact between two members normally separable by an intervening element, and particularly to methods of forming raised surfaces in at least one of the members of sufficient height to extend toward and contact the other member notwithstanding the intervening element therebetween.
2. Description of the Prior Art
In one process for the manufacture of flexible printed circuits, thin layers of copper are initially bonded or clad to opposite major surfaces of a flat, flexible, polymeric substrate of continuous length. Thereafter, a plurality of thin-layered images of a dry photopolymer material are deposited onto spaced portions of the substrate copper in selected patterns to mask some portions of the copper and leave exposed selected portions of the copper which define the patterns of the ultimate conductive paths supported on the flexible substrate. Other portions of the substrate copper adjacent to longitudinal marginal edges of the substrate also remain exposed.
The masked substrate is then passed through a copper electroplating bath to facilitate the plating buildup of copper onto the exposed portions of the thin layers of substrate copper previously bonded to the substrate.
In the electroplating process, a plurality of electrode guide rollers are positioned above the plating bath and cooperate with a corresponding plurality of guide rollers located within the bath to facilitate guiding the masked substrate in undulating, successive passes through the bath. A plurality of plating anodes are positioned within the bath and are connected to the positive side of a plating potential source. The electrode guide rollers are connected to the negative side of the plating potential source.
As the copper clad substrate is passed into contact with the electrode guide rollers during the plating operation, some portions of the exposed substrate copper must make contact with the electrode guide rollers so that the negative side of the plating potential source is coupled, through the substrate copper, to the plating bath to facilitate the plating process. However, even though the image patterns are thin, the intervening presence of the electrically nonconductive, dry photopolymer material generally prevents the exposed substrate copper, including those portions adjacent to the longitudinal marginal edges of the substrate, from making the necessary contact with the electrode guide rollers.
Due to normal tensioning of the substrate as it passes about the electrode guide rollers, portions of the exposed substrate copper adjacent to the longitudinal marginal edges of the substrate occasionally are drawn into contact with the electrode guide rollers. This results in a sporadic plating process. In order to compensate for the sporadic plating, the substrate is moved the plating bath at a slower speed and the plating current is increased. In this manner, some amounts of copper are plated onto the exposed substrate during the plating operation. Ultimately, the required buildup of copper onto the exposed substrate copper is accomplished but at the expense of a slower process speed and higher plating current.
However, as the substrate exits from the bath in the leading stages of the plating operation, small amounts of plating solution are carried from the bath on the exposed substrate copper. Since the intervening photopolymer material is preventing the substrate copper from contacting the electrode guide rollers, the substrate copper is effectively connected, through the bath and the plating anodes, to the positive side of the plating potential source. As successive portions of the substrate copper pass about the cathodic electrode guide rollers, the presence of the small amounts of plating solution between the substrate copper and the electrode guide rollers results undesirably in the plating of copper from the solution onto the electrode guide rollers. However, since the electrode guide rollers are made of stainless steel, and thereby have a passive surface, the copper is loosely plated onto the electrode guide rollers and sometimes appears as a copper-based slurry.
As trailing successive portions of the substrate copper pass over the copper plated, electrode guide rollers, loose bits of the copper which was previously plated onto the rollers transfer to and become embedded in the photopolymer material and the substrate copper. As the substrate is processed in subsequent solder plating, photopolymer-material removal and copper etching procedures to form flexible printed circuits, the embedded bits of copper remain in the developed circuits and form undesirable shorts between adjacent conductive paths of the flexible printed circuits.
Thus, the substrate copper must make contact with the electrode guide rollers to insure that the negative side of the plating potential source is coupled, through the substrate copper, to the plating bath to effect the proper and desired plating of solution copper onto the substrate copper.